This invention relates to optical multiplexing systems.
There are various techniques of multiplexing a number of optical devices (such as sensors) some of which are described in patent GB 2184910B. One technique which has recently been proposed is called coherence multiplexing and may take several different forms. In one form, radiation consisting of partially coherent wave trains is supplied to a first channel, such as an optical fiber. The first channel is coupled at points along its length to a second channel, such as a second optical fiber, so that a portion of the radiation in the first channel is coupled to the second channel at each point. At the second point and succeeding points, a part of the radiation on the second channel will be coupled back into the first channel. The path length between adjacent points is arranged to be different for the two channels so that the primary wave train, that is, the original wave train in the first channel, reaches the demultiplexing device at the end of the channel at a different time from the secondary wave trains produced by coupling between the channels. Each coupling point will cause additional secondary wave trains to be produced in both channels. An optical sensor, or similar device, is connected in one or the other of the channels, between the coupling points, so that a change in the variable being sensed causes a change in the wave trains in that channel. This arrangement produces a series array of Mach Zehnder interferometers. The demultiplexer is connected at the end of the second channel and takes the form of an interferometer which may operate either by comparing the primary pulse sequentially with each of the secondary pulses, or by comparing adjacent secondary pulses. This system, however, suffers from a high power budget loss and crosstalk.
An alternative arrangement for producing coherence multiplexing involves the use of a highly birefringent fiber in which the two interferometer arms of the Mach Zehnder arrangement discussed above are replaced by the two linear polarization modes of the fiber. Stressing the fiber at a point causes coupling between the two different modes. An interferometer can be used to identify the location of different points of stress along the fiber. This arrangement can have a lower power loss than the previous arrangement but still suffers from a relatively high crosstalk.